The following pertains to the nuclear reactor safety arts, nuclear power arts, and related arts.
A nuclear reactor includes a radioactive core comprising a fissile material immersed in coolant. In a light water reactor, the fissile material is typically a uranium composition such as uranium oxide (UO2) enriched in the fissile 235U isotope, and the coolant is purified water. The nuclear reactor core and the immersing coolant are contained in a reactor pressure vessel. A coolant flow circuit may be provided via large diameter piping, e.g. between the reactor pressure vessel and an external steam generator, or between the reactor pressure vessel and a turbine. For example, in a typical boiling water reactor (BWR), a coolant circuit is provided to transfer coolant in the form of steam to drive a turbine to generate electricity. In a typical pressurized water reactor (PWR), a coolant circuit is provided to transfer coolant to a steam generator. In integral PWR Designs, the steam generator is located inside the reactor pressure vessel, so that there is no external coolant loop implicating large diameter piping.
In a loss of coolant accident (LOCA), there is a radiological release outside of the reactor pressure vessel as escaping coolant flashes to steam. To prevent radiological release to the environment, the reactor pressure vessel is contained in a radiological containment (sometimes shortened to “containment”). A PWR with its external steam generator is located inside a radiological containment in the form of a steel or steel-reinforced concrete structure. This radiological containment is located in a compartment of a surrounding reactor building that services the nuclear reactor and ancillary components (sometimes also called a reactor service building). In PWR designs, the steam generator (whether external from the reactor pressure vessel or integrally located as in integral PWR designs) also receives secondary coolant that is kept separate from the (primary) coolant that flows through the reactor pressure vessel. This secondary coolant is therefore not contaminated with radiological contaminants, and may be piped outside containment through suitable safety valving.
A typical BWR nuclear island is designed similarly to a PWR. However, in a BWR coolant in the form of steam is piped directly into the turbine, which is located outside containment. (By contrast, in a PWR secondary coolant converted to steam and drives the turbine). This steam contains radiological contaminants. Accordingly, in some BWR systems a secondary containment is provided which surrounds the (primary) radiological containment and the turbine. The secondary containment is active, i.e. maintained at a negative pressure using active blowers to pull air through filters to the outside environment.
Some primary containment designs have leakage rates as low as 0.1% of containment volume per day, providing a decontamination factor over the first 24 hours after a radiological release of approximately 1000. A secondary containment can improve upon this, but requires AC power to operate the blowers and other active components. Secondary containment is difficult to employ in a passive nuclear power plant because safety-related AC power is not available. Even where safety-related AC power is available, it can be lost due to weather-related events or the like.